Molecular Dynamics Simulation of Ultrasonic Joining Mechanisms
| Leitung: | Dr.-Ing. Jens Twiefel |
| E-Mail: | mailto:twiefel@ids.uni-hannover.de |
| Jahr: | 2019 |
During ultrasonic (US) joining processes, microwelds are formed, deformed, broken and re-formed. The competition between formation and breakage determines the bonding quality. An example is shown in Abbildung 16. The wire part (upper part) slides on the substrate due to the vibration and continuous plastic deformation. During the movement, the surface topography of both the wire and the substrate significantly changes. The total microweld area is reflected in the shear stress change, as shown in Abbildung 17. It first increases and decreases as the edge of the substrate part is reached.
It is shown that the local positions where old microwelds have been broken still offer the opportunity for the formation of new microwelds, even with the same asperities. The constant horizontal movement of the wire significantly changes the surface topographies. Under the same conditions, a high stiffness of the materials has a negative effect on the microweld growth. A larger approaching distance or a larger deformation of asperities (especially large asperities) significantly helps the microweld growth. A large vibration amplitude makes the mi- croweld changes faster while a large vibration amplitude does not necessarily increase the shear stress and the microweld area.
US vibration significantly enhances the diffusion. For example, no diffusion occurs in a Cu-Al system at 300 K within 3 ns. When US vibration is applied, however, diffusion takes place. As shown in Abbildung 18, obvious diffusion of Cu into Al is observed, no matter whether the vibration is added in horizontal or vertical direction. The vibration causes large amounts of defects which greatly enhance the diffusion. The vibration amplitude and frequency significantly influence the process.
